This study focuses on the role of virus-specific proteins in the replication of Gross leukemia virus and other naturally occurring mouse leukemia viral variants, as well as on the genetic factors influencing viral replication. We plan to examine the expression of viral structural proteins and of other virus-related proteins, such as the GCSA and GIX antigens during single-cycle viral growth in tissue- culture. The studies will be carried out in both fibroblast and lymphoid cell cultures. We intend to explore the kinetics of synthesis of viral proteins and their cytological localization during viral maturation. From the genetic point of view, a dominant factor in the infective process in the laboratory, as in nature, is the interaction between the Fv-1 allele(s) of the host cell and the locus in the viral genome specifying N or B character. We intend to explore the molecular basis of restriction due to the Fv-1 locus, utilizing as a background the data obtained on the "normal" cycle of viral growth. Another genetically determined situation which we will compare to productive infections is the partial expression of virus-related functions (e.g., gs and GIX antigens) in cells of certain mouse strains.